While the level extraction of voice signals is a technique required in many fields of art such as, for example, echo suppression in a long-distance trunk circuit or the extraction of voice signals within a given time interval in a voice analyzer-synthesizer system, etc., the present invention will be described hereunder mainly in connection with the case of its application to an echo suppressor.
Generally, in telephone circuits both two-wire circuits and four-wire circuits co-exist, and at junction point therebetween a hybrid coil is normally employed. However, if the mismatching of impedances should exist there, then reflection would be produced, which results in an echo in a long-distance telephone circuit and causes troubles in telephone conversation.
Heretofore, as means for dealing with such echo, an echo suppressor which monitors signal levels in four-wire circuits and which breaks a circuit that is not in use or which provides a signal attenuation to it, has been widely utilized throughout the world. Since the echo suppressors are provided in quantities substantially equal in number to the number of the circuits, the number of echo suppressors becomes very large, and accordingly, concentration of the echo suppressors is required. In association with the concentration of the echo suppressors, high reliability and stability are required, and miniaturization is also required. Conventional echo suppressors are typically composed of analog circuits, and so, if they were to be concentrated by themselves on a large scale, a great amount of time would be spent for maintenance and adjustment thereof and their miniaturization would also be difficult. In accordance with the recent establishment of digital techniques, replacement of an analog circuit by a digital circuit having excellent stability and requiring no adjustment has been popularized; examples of such replacement are also evident in the art of echo suppressors.
Among the principle component elements of the echo suppressor, that element occupying the largest volume is a voice signal level detection circuit. Functions required for a voice signal level detector circuit to be used in an echo suppressor are that its detection speed is fast, and that once a level has been detected the detected level can be held for a predetermined period of time (for instance, several tens of seconds) without decaying abruptly, in other words, that the voice signal level detector circuit can have a hang-over time. The necessity for a hang-over time is due to the fact that a time delay although an echo path (from an output terminal on the receiver side to an input terminal on the transmitter side of an echo suppressor) is taken into consideration. Construction of a level detection circuit having the above-described functions and employing analog circuits can be realized in a relatively simple manner by employing an integrating circuit having different time constants for charging and discharging, respectively, provided that its stability and accuracy are disregarded. On the other hand, a level detection circuit for digitized voice signals in a digital echo suppressor in the prior art, is constructed of an integrating circuit and a counter circuit in combination. In such a case, miniaturization is hampered in that a large number of bits must be handled by the integrating circuit for obtaining the signal level at high precision, and that the counter circuit is needed. Furthermore, because of the need for integrating operations, it is difficult to enhance the detection speed.
Regarding voice signals, since their arithmetic mean values or root mean square (RMS) values can be deemed to be proportional to their peak values, it is possible to use a maximum value tracing circuit as the level detection circuit. By use of a maximum value tracing circuit, there exists a possibility of reducing the number of bits in a memory section (accumulator) in comparison to an integrating type of level detection circuit, while its response speed is very quick, and therefore, it is suitable for use in an echo suppressor.
An example of echo suppressors which make use of a maximum value tracing circuit was proposed in an article by E. Ferriello published on p.p. 1176- 1181 of IEEE Trans. Communications (Dec. 1972). The proposed apparatus is an echo suppressor comprising a peak level detector for discriminating a peak level into one of a plurality of multilevels at an interval of 2 dB, and gate signal generators for counting a hang-over time provided equal in number to the number of the discriminated levels. However, the apparatus according to the disclosure in this article includes a large number of elements such as counters, gatecircuits, and the like, and accordingly it is not deemed suitable for miniaturization.